THE GREEN ALGAE OF NORTH AMERICA 135 



6. VOLVOX Linnaeus, 1758, p. 320. 



Colonies spherical, of from 200-20,000 cells ; cells pyriform, 

 united by protoplasmic threads, and closely set near the surface 

 of the sphere, the cilia projecting. A small number of cells, 

 usually 8, are specialized as parthenogonidia, asexually form- 

 ing new colonies by repeated division. Sexual reproduction by 

 antheridia and oogonia ; the former containing 8-256 clavate 

 antherozoids, with red stigma and two cilia ; oospores round, 

 with smooth or stellate membrane ; new colonies formed by 

 their germination, in the same way as by parthenogonidia. 



KEY TO THE SPECIES OF Voi/vox. 

 r. Colonies over 500 /u diam., containing several thousand cells. 



T. V. globator. 

 i. Colonies not over 300 /u. diam., of less than 1,000 cells. 



2. V. aureus. 



1. V. GI.OBATOR Linnaeus, 1758, p. 320; Ehrenberg, 1838, p. 

 68, PI. IV, fig. i ; Wolle, 1887, p. 158, PI. CLI, figs, i-io, CLII, 

 figs. 1-9 ; P. B.-A, No. 1264. Colonies monoecious and usually 

 proteranclrous, 600-800 p. diam., composed of many cells, 3,000- 

 20,000; vegetative cells 2-3 /* diam. ; parthenogonidia to the 

 number of 8 formed in a colony, about 50 /u. diam. ; oogonia 

 20-40 in a colony, about 50 p. diam. ; oospores brownish, outer 

 membrane covered with stellate projections, inner membrane 

 thick, gelatinous ; antheridia seldom over 5 in a colony, globose, 

 35-40 p. diam., producing numerous antherozoids, 5-6 p. long. 

 Fig. 22. Mass., Mich., 111., Cal. Japan, Europe. 



One of the most striking of the fresh water algae ; the very 

 minute cells are united into spherical colonies, large enough to 

 be visible to the naked eye, and moving by innumerable vibra- 

 tory cilia, two to each cell. It seems to be generally distrib- 

 uted, and sometimes occurs in quantities in ditches and quiet 

 pools, and is found practically throughout the year. 



2. V. AUREUS Ehrenberg, 1832, p. 77; 1838, p. 71, PI. IV, 

 fig. 2; P. B.-A., No. 1176. Proterogynous ; colonies about 200 

 p. diam., composed of 600-900 cells; vegetative cells 4-6.5 //, 

 diam.; oogonia 1-15, 50-60 p. diam., outer membrane smooth, 

 oospore reddish-brown, 4.8-60 //. diam. ; antheridia numerous, 

 15-17 M diam., containing 16 closely packed antherozoids, 

 10-13X3-4 /* 111., Mich., Cal. Europe, Asia. 



A smaller plant than V. globator, and amply distinct in every 

 particular. The three classes of reproductive organs, partheno- 

 gonidia, oogonia, and antheridia, may occur in separate colo- 



